1. Field of the Invention
This invention relates to a process for starting up a wet oxidation system using essentially pure oxygen or oxygen enriched air for oxidation of a concentrated wastewater.
2. Information Disclosure Statement
The removal of pollutants from wastewaters by wet oxidation is a well known method of wastewater treatment. Pollutants are oxidized by oxygen from an oxygen containing gas in a closed system at elevated temperatures and pressures.
Operating temperatures range from 100.degree. C. (212.degree. F.) to 373.degree. C. (703.degree. F.) with pressures sufficient to retain water in a liquid state plus supply sufficient oxygen containing gas for oxidation of the pollutants. The system pressure typically ranges from about 100 psig (690 KPa) at lower temperatures to about 4,000 psig (27,578 KPa) at the higher temperatures.
An overall measure of pollutant content of wastewaters is Chemical Oxygen Demand (COD), the weight of oxygen needed to oxidize the pollutants in a unit volume of wastewater. COD is usually measured in units of grams per liter. The objective of wet oxidation generally is to remove as much COD as possible from the wastewater. COD removals of 85% to 99+% can be attained at the higher oxidation temperatures.
The start up of wet oxidation systems treating dilute liquid wastes with a COD of about 10 g/1 or less with air, oxygen enriched air or essentially pure oxygen gas can be accomplished without difficulty. The pressurized liquid and gaseous phases are mixed and heated, flow through a reactor vessel, are cooled, depressurized and then separated into an oxidized liquid phase effluent and a noncondensed offgas phase. The heat generated by the oxidation of the dilute waste is not so great as to cause control problems when raising the temperature of waste and oxygen containing gas from ambient to the selected wet oxidation temperature.
Wet oxidation of more concentrated liquid wastes, with COD in excess of about 10 g/1, using essentially pure oxygen gas or high oxygen content gas produces potentially difficult to control situations, particularly when starting up the wet oxidation system. The large amount of heat liberated on wet oxidation of the concentrated wastes with high oxygen content gas makes system startup and operation most difficult to control. Well developed operating procedures are required for such a wet oxidation system. Bauer in U.S. Pat. No. 4,369,115 discloses a safe method for mixing pure oxygen, or an oxygen enriched gas, at high pressure, with waste liquor in a wet oxidation system, such that intermittent drying conditions are avoided, preventing possible spontaneous combustion, fire or explosion. The oxygen is added to pure liquid water before it is mixed with the waste liquor.
Bauer et al. in U.S. Pat. No. 4,384,959 describe a continuous wet oxidation process using oxygen or oxygen enriched gas. An inert diluent gas is injected into the aqueous liquor or reactor off-gas such that oxygen in the gas phase is diluted by the sum of generated water vapor, produced carbon dioxide and injected inert gas to a concentration less than required for spontaneous combustion at every location in the system having surfaces not continually exposed to continuous liquid water phase.
Chowdhury et al. in U.S Pat. No. 4,395,339 describe a method of operation for pure oxygen wet oxidation systems where potential for oxidation is established prior to introducing oxygen and maintained until after oxygen flow is stopped. An inert gas, either from an external source or generated in situ, is present in the reactor from before oxygen flow is introduced until after oxygen flow is stopped, and the oxygen introducing means is clean for oxygen use from prior to the introduction of oxygen flow until after the oxygen flow is stopped.
Applicants have devised a startup procedure for wet oxidation systems using essentially pure oxygen gas for the oxidation of concentrated wastewaters which provides safe and controlled operation of the system.